An ISM frequency band is an unlicensed frequency band allocated for industrial, scientific and medical use which utilizes weak electric field intensity. Recently, since a usable frequency band is decreased due to an increase in a use and a popularization of a wireless communication device, an attempt to utilize the ISM frequency band which is the unlicensed frequency band has been increasing. Particularly, a utilization of a wireless communication using the ISM band is actively considered as a replacement means to solve the depletion of available frequency bands.
Due to the above reasons, many equipment and devices are used in the ISM band without supports from network operator for network management and its operation. However, an interference caused by a user who arbitrarily installs and uses various equipment and devices is problematic, in the case of occurring communication interference, and brings down performance degradation and even worse, equipment malfunction.
Among frequency bands of 2.4-2.4835 GHz and 5.725-5.85 GHz which are expected to be rapidly increased in the near future in the ISM bands, although there is a serious amount of RF activities and noise power in a 2.4 GHz band in particular, the number of communication devices using the corresponding band is continuously increased. In addition, the number of communication devices using 5.8 GHz band such as an ITS (Intelligent Transportation System) and a high speed wireless access network is rapidly deployed. Therefore, there is a high possibility that serious problems such as the equipment malfunction and the communication interference may occur because various equipments are operated without any control within the ISM frequency band.
On the other hand, a broadband wireless LAN only allows a limited number of channels so that interferences between access points is apt to be generated when the access points are installed and used arbitrarily. For example, in the case of a DSSS (Direct Sequence Spread Spectrum) of IEEE 802.11 and 802.11b, although a use of 11 channels are allowed (in the U.S.) in the 2.4 GHz band, a frequency range between center frequencies in the bands should be separated wider than 25 MHz in order to prevent an interference between channels. Therefore, only three non-interference channels can be used. Even if IEEE 802.11a uses a 5 GHz U-NII (Unlicensed National Information Infrastructure), eight non-interference channels for indoors and four non-interference channels for outdoors are allowed.
Currently, when a wireless LAN is set up to provide a broadband wireless LAN service within a communication coverage limited to a predetermined region, a sufficient number of wireless LAN access points should be installed and an overlapping frequency should not be allowed between the access points in order to provide an uninterrupted service. However, an allocation of limited number of the non-interference channel and a frequent use of communication equipment utilizing an unlicensed frequency (ISM or U-NII) may result in a degradation of a quality of the broadband wireless LAN service due to an uncontrolled channel access induced by an arbitrary wireless equipment user in a surrounding environment.
Moreover, when a frequency having higher frequency band than 2.4 GHz and 5 GHz is used in order to increase a data transfer rate higher than the current broadband wireless LAN, a large radio wave shadow region may be generated by ambient obstacles existing in the line of sight. More broadband wireless LAN access point should be used in order to supplement this, resulting in an increase of the interference due to an overlapping of used channels between access points.
In the case of a cellular network, an accurate cell planning is carried out, considering various variables such as a fading due to a topology characteristic and an artificial structure, radio wave interference due to a multiple path and the number of subscribers who will place a call in a corresponding cell. The entire design, construction and management regarding to these tasks are carried out by radio engineering professionals. However, contrary to the cellular network, in the case of the construction and management of the broadband wireless LAN, it is common that a LAN based on a wire is simply replaced by the wireless LAN. Particularly, such designing, construction and management of the wireless LAN network are commonly carried out by general users or an IT manager of an who has little knowledge on radio engineering.
Moreover, the access point of the broadband wireless LAN and a broadband wireless LAN card are manufactured to be easily used by a general computer user. Therefore, only basic values of network equipment are commonly preset so that the general user can install without an understanding of a sophisticated internal working mechanism, thereby excluding a professional process such as preventing interferences between corresponding equipment and card and other equipments.
Such a simple designing and construction of the network without considering a cause of interference due to the use of the unlicensed frequency may result in following serious problems. For example, in the case of an office or a multiple dwelling unit (MDU), when another user installs his/her own wireless LAN an uninterrupted wireless LAN service is obstructed due to the interferences with the existing wireless LAN users or users who will install and use the wireless LAN later.
Aside from such frequency interferences between wireless local area networks, when the sufficient number of access points are not installed in the process of a BSS (Basic Service Set) planning, isolated BSSs such as BSS1 and BSS2 shown in FIG. 1 may occur. Here, a moving station STA1 cannot establish a connection to the wireless LAN from a moment the station STA1 is out of the first BSS BSS1 until a moment the station STA1 reaches the second BSS BSS2, resulting in a problem of an interrupted communication. In addition, a direct relay of a message is not possible between stations STA1 and STA2 located in the first BSS BSS1 and stations STA3 and STA4 located in the second BSS BSS2.
Moreover, in the case of a single BSS, when the number of stations connecting to an access point in the corresponding BSS increases, a problem of a communication interruption or a degradation of a communication quality due to an excess of a network capacity may occur.